Ideally you will never leave your fish tank for more than 2 days; just kidding. This really should be no problem. Every system should be more or less predictable in it's upkeep.

That is, you should know exactly what it needs and at what frequency. The best way to figure out how to fine tune your husbandry regimen is to take notes and monitor your system. This way, when the times comes that are not around to care for the tank, you may impart some of the knowledge you have acquired on to some lesser being who is not yet an aquarist.

The simplest and best approach to doing this is to make very clear and detailed instructions and to assume that the person doing the work knows absolutely nothing about aquariums.

So just make sure to have some easy to follow instructions to leave your fish-sitter and everyone will be happy.